When the Earth’s mantle fills the water!

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[EN VIDÉO] How is magma formed before a volcanic eruption?
Magma is the origin of the formation of volcanoes. This molten rock, which erupts in the crater of the volcano, comes from the partial melting of the earth’s mantle. Futura met Dr. Jacques Marie Bardintzef in volcanology who tells us about the origin of magma.

One of the main mechanisms involved in tectonic plates he is here subductionwho sees lithospheric plate Pass under another plate to be recycled inside Coat. Subduction zones are generally associated with intense volcanoes, often of the explosive type. In addition, most of the current mountain ranges, like the Alps Or the Himalayas, as a result of a continental collision that followed the closure of an ocean by a subduction zone. Thus, these mountain ranges contain many clues that allow us to indirectly study the complex mechanism of subduction zones.

The mantle absorbs large amounts of water in subduction zones

Many parameters are triggered and their diversity makes each subduction zone a unique dynamic system.

One of these factors is water. It is indeed an essential element that will directly affect the mantle fusion ability and Shellbut also behavior the silhouette while going up and Formation of different mineral deposits.

It has long been known that subduction zones represent high humidity environments. When submerged in the mantle, the fused plate carries an enormous amount of sediment Saturated oceans. This water will significantly moisten the mantle rocks and directly affect the formation of magma. The volcanic rocks Issued by volcanoes On the roof bear the signature of this wettingwhich will also define the style eruptive. more of lavas It is more moisturized Explosive explosions. This explains why The most destructive eruptions generally occur in subduction zones.

Finding the initial water content of magma is a complex problem

So far, analysis of igneous rocks indicates that, in this subduction context, the mantle contains about 4% water (per unit weight). However, a new study questions this value, and puts forward the idea that the signature in igneous rocks would not represent the actual water content of the primary magma. For the authors of the study published in natural earth sciencesIt does seem likely that some processes – either at the time of the rapid rise of the magma, or as it reaches the surface – somehow mask the original water content of the magma. liquid Magmatic.

To try to find the exact value of the amount of water, the research team was interested in a specific type of water igneous rocksthey Plutos, which unlike igneous rocks do not crystallize at the surface but deep in the crust. A new analytical method has also been developed to study the composition of underground rock samples from Himalayas and updatesbump According to the tectonic deformation of this massive collision zone.

A coat more moisturizing than we thought

Thus the crystals of these rocks show much less signs of alteration than igneous rocks exposed to the elements at the surface. Their results eventually show that it is not 4, but more than 8% of the water (by weight) that the magma had before it crystallized.

These results also provide a better understanding of sediment formation copperor gone orsilverwhich requires a large amount of water. 4% previously estimated was not sufficient to explain the formation deposit From metal. The problem is now resolved. These ore deposits would thus be formed from volcanic fluids very rich in water (12 to 20%) which would have separated from the primary magma at the time of its crystallization. So understanding the generation of these superhydric rock fluids is of great interest to the search for new deposits.

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